Non-bulking activated sludge process

ABSTRACT

A liquid stage, or its hydraulic equivalent, is maintained in an activated sludge system where incoming waste water and recycle sludge are initially mixed. Both the dissolved oxygen and the steady state BOD are maintained at or above minimal concentrations critical to promote the selective growth of active, rapidly settling, non-filamentous biomass thereby inhibiting or precluding the development of filamentous biomass of higher surface area.

tlnited States Patent 11 1 1111 3,864,246 Casey et a1. 1 1 Feb. 4, 1975 [54] NON-BULKING ACTIVATED SLUDGE 3,547,814 12/1970 McWhirter 210/7 PROCESS 3,547,815 12/1970 McWhirter 210/7 3,725,258 4/1973 Spector et a1 210/7 [75] Inventors: Jeremiah P- Cas y; Cu i 8- 3,764,524 10/1973 Stankewich 210/5 McDowell, both of Macungie, Pa.; Marshall L. Spector, Belle Mead, Primar y Examiner-Thomas G. Wyse Alan Zupko, Malvem Attorney, Agent, or Firm-Bernard M. Weiss; Barry [73] Assignee: Air Products and Chemicals, Inc., Moyerman Allentown, Pa.

22] Filed: Jan. 24, 1973 ABSTRACT [21] APPL 77 A 1iquid stage, or its hydraulic equivalent, is maintamed 1n an activated sludge system where incoming waste water and recycle sludge are initially mixed. [52] U.S. C1. 210/7, 210/15 Both the dissolved Oxygen and the Steady State O [51] hit. C1. ..C02C 1/06 are maintained at or above minimal concentrations 158] Fleld of Search 210/51 15, 41 61 critical to promote the selective growth of active, rap- 210/11 14 idly settling, non-filamentous biomass thereby inhibiting or precluding the development of filamentous bio- 156] References C'ted mass of higher surface area.

UNITED STATES PATENTS 3,547,813 12/1970 Robinson et a1 210/7 5 1 Draw F'gure REFERENCE MAX/MUM SPEC/76 OXYGEN UPTAKE RATE (M50017) 1% A5 A FUNCTION OF TEMPERATURE 1 1 l 1 l 1 l 1 1 1 ,0 l l I l I an 3.34 3.59 3.42 3146 3.50 3.54 a.

NON-BULKING ACTIVATED SLUDGE PROCESS BACKGROUND OF THE INVENTION I 1. Field of the Invention This invention relates generally to improvements in the treatment of municipal sewage and/or industrial waste water by the activated sludge process. It is particularly concerned with control of operating conditions to enhance the selective production and maintenance in the system ofa highly active biomass essentially free from filamentous growth, whereby the obtained sludge has favorable settling characteristics.

2. Prior Art The activated sludge process has been used for many years for the removal of biological oxygen demand, BOD, from waste water. This process consists of maintaining an aeration basin in which waste water is fed to a suspension of microorganisms to form a mixed liquor. The mixed liquor is aerated to furnish oxygen for the respiration of the biomass, which sorbs, assimilates, and metabolizes the biological oxygen demand of the waste water.

After a suitable period of aeration, the mixed liquor is introduced to a clarifier in which the biomass settles and the treated waste water overflows into the receiving stream. A major portion of the settled biomass, which is concentrated at the bottom of the clarifier, is recycled to the aeration basin and a minor portion is purged in order to maintain a constant biosolids inventory within the system. This process has been extensively described in the literature and several of its modifications summarized in a special report on Waste Water Treatment" by R. H. Marks contained in the June 1967 issue of POWER.

Despite the versatility and effectiveness of this process and its many modifications, there remains a major problem. It is the proliferation of high surface area and- /or filamentous species, such as Sphaerotilus, which do not settle adequately in the clarifier. Thus one consequence of filamentous biomass is the inability to disengage the biomass from the treated waste water. Another is the inability to concentrate the biomass adequately at the bottom zone of the clarifier, thus necessitating a high recycle volume from the clarifier relative to the volume of influent waste water flow, in order to maintain an adequate concentration of biomass or mixed liquor volatile suspended solids (MLVSS) in the aeration basin. This results in theneed for large aeration basins and high capacity sludge recycle pumps and lines.

Once established, filamentous microorganisms are difficult to displace short of drastic action, which is generally harmful to the total biomass. In any case, activated sludge treatment plants usually experience periods of prolonged unsatisfactory performance when this bulking occurs.

Several modifications of the basic activated sludge process have been proposed in an attempt to avoid or overcome this deficiency. One method is to distribute the influent waste water to different sections of the aeration basin in order to spread out the oxygen demand. Another is to decrease the loading of BOD to the aeration basin. A third method is to add poisons to the system in order to selectively kill off the high surface area filaments. Still another method is to make the system anaerobic and thus kill the filamentous biomass which is composed largely of obligate aerobes.

None of the above methods is satisfactory in that they do not preclude the problem of filamentous bulking."

It is therefore among the objects of the present invention to provide a process which avoids proliferation of filamentous biomass by promoting the growth of more desirable species. In addition, the process of the invention produces an active, dense biomass, which requires minimal oxygen to remove BOD.

SUMMARY OF THE INVENTION It has now been found that the desired selective production of a non-filamentous biomass population of highly active dense and rapidly settling microorganisms can be promoted and sustained by maintaining controlled conditions during the initial stage of aeration of the aqueous waste in an activated sludge system, whereby the growth and proliferation of undesired higher surface area microorganisms having poor settling characteristics is minimized. This is accomplished in accordance with the invention, by providing at the region where waste water is admitted to an aeration basin a separate initial liquid treating stage, or its hydraulic equivalent, as determined by the residence time distribution via tracer experiments (see Levenspiel, Chemical Reaction Engineering, John Wiley & Sons, N. Y. 1962, pp. 242-308) and maintaining therein a sufficiently high level of soluble BOD and an adequate content of dissolved oxygen. Thereafter the BOD- containing liquid from this initial treating stage without intermediate clarification is subjected to further aeration in one or more stages to assimilate and oxidize the BOD largely sorbed in or on the biomass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An initial liquid treating stage is where BOD in influent waste water is distributed to a mixed culture of biomass. The present invention provides for an initial liquid stage in which critical conditions of high dissolved oxygen and high concentrations of BOD are maintained. The latter is achieved by maintaining a high food to biomass loading, F/M, throughout this initial treating zone. The microorganisms which compose filamentous biomass, having a relatively high surface area, are thus deprived of the conditions which make them most favored to proliferate because of their large surface area, relative to that of more desirable species.

Conditions which favor the production of undesired filamentous, high surface area biomass are:

1. An abundance of soluble BOD in the presence of low levels of dissolved oxygen.

2. An abundance of dissolved oxygen in the presence of low levels of BOD.

3. Low levels of BOD and low levels of dissolved oxygen.

Conversely, conditions which maintain both an abundance of soluble BOD and an abundance of dissolved oxygen favor those microorganisms which can assimilate and oxidize BOD rapidly and consequently grow more rapidly. In addition to having high assimilative and oxidationrates, these microorganisms are characterized by having a low sludge volume index (Mohlmann) and high zone settling velocity (ZSV).

The species which assimilate and oxidize BOD most effectively will eventually dominate the biomass population. Thus the method of this invention promotes the development of desirable active species to the competitive disadvantage of undesired filamentous species.

It is well recognized (Advances in Bio. Waste Treatment," p. 299, Eckenfelder & McCabe, MacMillan, 1963) that the major portion of BOD is removed from mixed liquor within the first 5 to minutes contact time. Therefore, the allocation of food energy, coupled with an adequate supply of dissolved oxygen in these critical 5 to 20 minutes, determines which species will dominate the mixed culture. The population dynamics are largely established in this initial zone. Therefore, the conditions maintained in this zone are critical to determining properties of the activated sludge.

Further aeration after this initial contact is necessary to complete oxidation and assimilation of the soluble BOD, which must necessarily be maintained at a high level in this initial stage according to the invention. The need to provide adequate liquid-solids mixing and certain levels of dissolved oxygen to operate activated sludge systems effectively has been well recognized in the literature. It has been suggested (POWER, op. cit., page 8-7) that the maintenance of at least 0.5 ppm of dissolved oxygen is necessary to avoid anaerobic metabolism within a fiocculant biomass. These conditions are also required in downstream aeration after the initial liquid stage of the present invention.

The necessity to maintain a minimal dissolved oxygen level as a function of the loading to an initial mixing zone to avoid the onset of filamentous growth has not heretofore been recognized. The need to maintain simultaneously a high BOD, or food concentration in the presence of an abundance of dissolved oxygen in an initial liquid stage in order to achieve good sludge properties, also has not heretofore been realized.

The BOD concentration in the mixed liquor within an aeration basin cannot be accurately described by the BOD concentration in the clear liquor alone, because the true concentration in the mixed liquor must include the BOD sorbed on and within the cell walls of microorganisms as well as that free in solution. It has been found possible to describe the minimal BOD concentration necessary to avoid growth of filamentous and/or other high surface area organisms in an initial mixed liquor phase, in terms of pounds of BOD fed per day per pound of biomass under aeration in this first mixed liquor stage, being a modified form of the F/M expression heretofore known, but wherein both the terms F and M are defined as discussed below.

We have found that the pounds of total soluble BOD fed per day is a useful measure of F. This is in distinct contrast to the use of BOD which is most commoly used, but which includes both soluble and insoluble BOD matter. Only soluble BOD is used in defining the process conditions employed in this invention because the holding time in this initial mixed liquor zone is too brief to allow much, if any, of the insoluble BOD to solubilize and thus participate in the food energy allocation in this zone. Soluble BOD does not fully measure the total soluble BOD. Soluble BOD- is a better measure, but determination of this value is too time consuming for practical application. It is reported in the POWER reference that essentially complete biological oxidation of organic matter takes about 20 days and the standard five-day, BOD equals about two-thirds of the total BOD. Therefore, an acceptable approximation for total soluble BOD may be calculated by multiplying the soluble BOD by a factor of 1.5. The pounds of soluble BOD fed per day and multiplied by a factor of 1.5 is used to define F, in the present specification and claims. To determine soluble BOD, the soluble matter is separated from insolubles by filtration through a 5 micron filter.

The term M is usually defined as the pounds of mixed liquor volatile suspended solids (MLVSS) under aeration in a given volume of mixed liquor. The MLVSS concentration in mixed liquor is determined by standard methods. In the present disclosure the term M, is used to define the pounds of MLVSS under aeration in a given volume multiplied by an activity coefficient, a. The term M, thus describes the activity of MLVSS acclimated to any specific waste water. The activity coefficient, a, is determined by measuring the maximum specific oxygen uptake rate, expressed as milligrams of oxygen per gram of MLVSS per hour, and then dividing this observed rate by a reference rate expressed in the same terms, displayed as a function of temperature in the FIGURE of the accompanying drawing.

The maximum specific oxygen uptake rate is experimentally determined by increasing the BOD loading to an acclimated biomass in the presence of abundant dissolved oxygen until the specific uptake rate no longer increases with increased loading. The method of determining the oxygen uptake rate is described in the literature by many authors. The one described in Dynamic Measurement of the Volumetric Oxygen Transfer Coefficient in Fermentation Systems" by B. Bandyopadhyay and A. E. Humphrey published in'Biotechnology and Bioengineering, Vol. IX, pages 533-544 (1967), is typical. The experimentally observed maximum specific oxygen uptake rate is divided by the reference maximum specific uptake rate indicated at the corresponding temperature in the FIGURE to arrive at the activity coefficient, a, which is a dimensionless number.

The activity coefficient also is an indicator of change of activity as a function of change in operating conditions. This will be illustrated in examples cited below. Unless otherwise noted, all food to biomass values cited below and in the claims of this patent application are in terms of F, and M as described above, wherein I indicates 1.5 times the weight of soluble BOD fed per day and M represents the active portion of the biomass obtained by multiplying the weight of MLVSS by the activity coefficient a determined as above described. When the teachings of this invention are followed and toxic chemicals are not present to a significant degree, the activity coefficient, 0:, will approach unity and M,, will be approximately equal to M.

The process of this invention concentrates on the importance of maintaining prescribed conditions in an initial zone of aerating mixed liquor, which are critical to obtain superior sludge properties. In practice of the present invention at least one additional stage of oxidation in the aeration basin will be necessary to complete oxidation plus assimilation of the BOD values remaining in solution and/or sorbed in the initial stage. The utilization of at least one additional aeration stage thus distinguishes this modification from activated sludge systems employing a single aeration zone, as in contact stabilization" or complete mix" systems.

It is during this further oxidation that sorbed particulate BOD is slowly hydrolyzed to produce soluble BOD. The oxygen demand response to the BOD solubilized in this manner is low in comparison to that required in the initial liquid treating stage in which the influent waste is contacted with recycle sludge, and usually can be characterized as a pseudoendogenous respiration rate. By following the principles of this invention, at- 5 tainment of the onset of an endogenous respiration rate may be achieved within a (raw influent only) aeration residence time of less than two hours. However, to achieve this condition an appropriate concentration of active MLVSS must be used. When endogenous respiration is approached or attained, assimilation of BOD has been sufficiently completed in the aeration basin, at which time the mixed liquor may be introduced to the clarifier.

A guide to selecting the proper concentration of acl5 tive MLVSS to treat various levels of soluble BOD concentrations of waste water within a raw influent only residence time not exceeding two hours, is presented by way of example in Table 1. This table lists residence In the practice of the present invention the initial liquid stage may be subdivided into smaller distinct zones. The minimal residence time in an initial liquid mixing stage should be interpreted to include a multiplicity of smaller zones, providing that the FJM of the mixed 1iquor contained in all such zones falls within the defined limits. If desired, an initial mixed liquor zone may be provided at each location within an aeration basin at which influent waste water is initially introduced.

The present invention is illustrated by the following non-limiting examples.

EXAMPLE I An Abundance of BOD in the Presence of Low Levels of Dissolved Oxygen time in an initial mixed liquor stage as a function of the F,,/M,, ratio maintained in this stage. The ranges of these variables cited in this table are for the purpose of estimating the level of MLVSS to be selected for treating a given strength waste water, rather than for limiting the scope of this invention.

With reference to Table 1, an MLVSS level which requires a raw influent residence time in the initial zone of less than 0.3 hours will, in general, attain a state of actual or pseudo endogenous respiration within two hours nominal residence time. Longer residence times at lower MLVSS are also within the scope of this invention. The values cited in this table may be interpolated or extrapolated to other values of soluble BOD and active MLVSS without departing from the scope of this invention.

An activated sludge system was established in which an uncorrected MLVSS level of about 3,000 ppm was acclimated to a waste water which was treated in a fivestage aeration basin. The first and second mixed liquor stages were each four liters and the last three mixed liquor stages were each eight liters in volume.

TABLE 1 RAW INFLUENT RESIDENCE TIME" (in hours) in INITIAL LIQUID STAGE as a FUNCTION OF F,,/M,, in INITIAL STAGE. SOLUBLE BOD CONCENTRATION and CONCENTRATION OF ACTIVE MLVSS 1.5 X SOLUBLE BOD, (ppm) 100 200 300 400 FJM 1st Stage 38 5.7 7.6 3.8 5.7 7.6 3.8 5.7 7.6 3.8 5.7 7,6

MLVSS at Standard Activity" (ppm) Time al es in Table are based on zero sludge recycle. In order to obtain the nominal residence time for any volumetric sludge recycle rate. R. the values in this Table must be multiplied by the ratio O/()+R where 0 ts the volumetric influent waste water flow rate.

"As defined in FIG. 1.

TABLE 2 OPERATING CONDITIONS 1.5 X Soluble BOD Sludge Nominal BOD Fed Removed Recycle Run MLVSS Residence Cone. Grams Filtered Effluent Temp. Vol.% of

No. ppm Time thrs.) ppm per Day Basis "C lnfluent TABLE 3 SLUDGE CHARACTERlSTlCS AS A FUNCTION OF DlSSOLVED OXYGEN LEVEL Dissolved FJM i/ u Oxygen Level, (M is (Mu is Max.Activi1y ppm in each Fila- SVl. Uncorrected Adjusted of Biomass Activity Run Run stage in order mentous Mohlmztn for a) lst for a) lst MgO Ig/hr Coeffi- Duration, No. of stage no. Growth lndcx Overall Basin Overall Basin MLVSS cient a Days 1 10.10.10.10.10 No 40 0.92 7.4 1.1 m 68 0.80 25 2 10.10.2.2,2 No 40 0.7a 6.2 1.2 9.4 62 0.66 29 3 N 411 11.93 7.5 1.9 15 46 0.50 18 4 No 55 0.x7 (to 2.11 is 40 0.43 31 Incipient on 1.24 14 5.3 41 30 0.34 s a Yes 175 1.3 10 3.4 26 33 11.514 x 7 Yes 250 1 (1 7.4 1.9 14 45 0.52 23 *lnflucnl B()D,, Increased from I98 to 4.13 p m Return to normal 800,, Load (I98 ppm The activity coefficient of the MLVSS listed in Table EXAMPLE ll 3 Obta'hed by dlvldhlg the maxlmum Spec'fic Oxygen An Abundance of Dissolved Oxygen in the Presence of uptake rate reported in Table 3 by the value for the ref- Low Levels of BOD Nitrifying System Frehce Specific Xygeh uptake rate at h correspond Two activated sludge units were established. They ih glg s h g; the graph m were duplicate units operated in identical fashion. The T S t erehce oxygen per gram 0 liquid staging for each was arranged to contain five R f if 3 b th uh I stages. The first four stages were each one-sixth and the hf o a e may Seen a 6 dc 25 fifth stage was one-third of the total volume (l/6, l/6, coefflcient is reduced as the dissolved oxygen level 1s H6 H6 V3) The MLVSS was maintained at abou lowered and increased the dissplved oxygein level is 2,560 ppm and the activity coefficient at 20C was 0 8 increased. The pr1me point of th1s example is to note for both systems The overall F [M exclusive of that an increase in the Mohlin'ann Sludge volume? Index monia oxidation demand was abbut d 85 and the F /M (8V!) and onset of proliferauon of filamentous biomass in the initial liquid stagewas about was triggered by operating at a hq f oxygen 3: Both systems were operating at 20C in a mode which z gg h l z g i fig gf gl gg g gfi effected over 90% removal of BOD (SOlublC plus insolm did romote filamentous g when the uble) and over 90% oxidation or assimilation of ammopp h g nia values. The soluble BOD was-introduced at about F IM to the 1n1t1al basin was lg (Run No. 4)t.fln other 190 pp and the ammonia at about 50 pp The y words the DO content in Run 0. 5 was insu icient to en l el h l at PM to avoid the onset of filamentous microorganism abundance :fpdiSSOlved en g growth'lporlthifs hbigh t ra-tiolfabqui a dis-solghd In order to demonstrate t h e effect of changing F /M oxygen eve o 21 out ppm 1s t e mmima require in 40 B a the initial treating stage. As a pr]2l1ctic)al gtlijide, the mi E li i 22 3 3223122 6iaieun gzglelvgzirgglirzgfdtgl tfilf:22 mum D leve (in parts per mi ion to e maintaine 8 no than 0.1 times the F IMH ratio in t at stage. eve of at least 2 ppmis advocated at even low F, /M,. ratios gzhg gig s g (f/ i g gy gl gf 3 g? g lhg to safeguard against possible sudden upset n the syssecond unit w emodf d t 1 t j d 'f h tem that might otherwise prove d fficult orunposslble t t v l l 0 "3 O t e to correct. Operation at F lM ratios 1n the 1n1t1al basm 0 a 0 h i d e b": 5 age Z 6 ast t rec Stages 4O of .1: than 4 m are not rec u e in t e ractice o t e principlgg of the press!" invention p F /M to the first stage of the first unit to about 10 and Referring again to Table 3 it may be seen that the to decrease the F,/M to the first stage of the second 1 tivity coefficient increased upon lessening the F /M 2 1 12 The overah Fl/M" was unaltered at ratio in Run 6 and even more so u on increasin the or o Sys dissolved oxygen level in Run 7, bu t the infestatic m of The Whh f ihcrfiased q l the hhhal filamentous biomass continued to exist over the addih Per (med as he whh ho slghlhcaht change tional period of over one month during which these In either sludge volume index or zone settling velocity, runs were conducted which had been and still remained at about 90 SVI and This difficulty in correcting the situation illustrates 5 to Q P hour SV, respectively. The activity the importance of maintaining conditions at the outset coefficleht lhcrehsed' to which preclude significant infestation of filamentous The Secohd hhh the decreased F s/Ma the growth. Run 7 was not continued for a sufficient length h F h tY a Poorly Sehhhg blomass popula' of time to provide for recovery of desued sludge propg i l z gv ci days- The Svl lhcreafed to over 200 erties, although the increased act1v1ty coefficlent obah t e teased to less than 2 Per hourtained is indicative that recovery would ultimately be 65 The Svl lhcreased to Over 300 Oh the Shah The achieved. Such complete recovery of desired sludge properties has been observed in other studies made in practice of this invention.

tivity coefficient decreased to 0.7.

This example demonstrates that an activated sludge system can oxidize ammonia and remove BOD effectively and at the same time produce a rapidly settling, dense sludge if the F /M to an initial aeration zone is as high as five. Conversely at an HIM of 2 /2 to an initial aeration zone, despite sufficiently high level of DO, sludge properties become unacceptable. This example illustrates the criticality of F,,/M,, in an initial liquid stage, as contrasted with the FJM ratio in the entire aeration basin.

EXAMPLE 111 An Abundance of Dissolved Oxygen in the Presence of Low Levels of Soluble BOD-Non-Nitrifying System at 14C A series of runs were made in which a waste water containing soluble BOD was treated in an activated sludge system at a nominal residence time of 1.6 hours (total flow) at about 14C. The dissolved oxygen level was maintained at about ppm in all stages of all runs. The MLVSS was maintained constant at about 3,000 ppm and the overall F,/M,, was maintained at from 1.4 to 2.0. The F,/M,, in the initial stage was decreased by changing the total number of equal liquid stages from 10 to 5 to 2 and finally to 1. The net effect was to decrease the F /M in the initial stage from 14 to 8.3 to 3.8 to 2.3. The nominal residence time in the initial mixing zone was increased at the same time from 0.16 to 0.32 to 0.81 to 1.6 hours.

TABLE 4 increased oxygen utilization, upon decreasing the F /M in the initial stage from 14 to 2.3, was from 0.58 grams oxygen per gram of BOD removed to 0.70 grams of oxygen per gram of BOD removed, respectively. This 20% increase in oxygen requirement is unexpected and represents a significant penalty in terms of oxygen transfer energy for the single stage system.

The main advantage in operating with high F,/M,, in the initial zone in these systems is evidenced by the superior sludge properties and avoidance of filamentous or other poorsettling growth. In runs where the initial F /M was 8.0 and 15.1, the SVI was in the range of from to 60 and the zone settling velocity was a remarkable 15 feet per hour.

EFFECTS OF CHANGING LlQUlD PHASE STAGING Run Duration 22 28 25 (days) No. Liquid Stages 10 5 2 MLVSS (mg/l) 3023 3230 3002 Filtered lnlluent l.5XSnl.B()D;,(mg/ll 316 358 363 Filtered Effluent (mg/l1 3.9 5.4 43

Residence Time overall (hrs) 1.61 1.61 1.63 Recycle 72 lnfluent 13.) 12.7 11.5 F /Ma -1st stage 14 8.3 3.8 F,,M,, -overull 1.38 1.7 2.0 71 BOD, removed in clarifier 96 95 95 71 BOD removed at 10071 clarificr efficiency 99 98 99 SVltml/g T) 52 51 53 start to finish to to 53 to ZSWft/hr) l5 15 19 start to finish to 15 to 19 to 6 O ./BOD,, removed wt/wt 0.58 0.64 0.63 VSS sludge in recycle (mg/l) 24800 28200 29200 Temp. C 13.6 13.7 Max. O- uptake mgO-Jg MLVSS/hr 44 40 41 Activity Coefficient 1.0 091 0.84

The grams of oxygen required to effect the removal of one gram of BOD increased as the F,/M in the initial mixed liquor stage was decreased. The extent of the OPERATION WlTH MUNICIPAL WASTE WATER EXAMPLE IV The summary of an example of operation of this method with municipal waste water is presented in 65 Table 5. The aeration basin of this system was divided into six liquid stages which were Vs, /tz, Vs, 2/8 and 2/8 of the total volume.

The system was essentially free of filamentous growth and exhibited excellent sludge settling properties as 1 1 may be evidenced from the low SVl, high ZSV and low sludge recycle volume. An endogenous oxygen uptake rate of 19 milligrams per gram of MLVSS was attained in 1.34 hours in this example.

TABLE 5 EXAMPLE OF OPERATION WITH MUNICIPAL WASTE WATER MLVSS Aeration Basin (mg/l) l7l5 MLSS Recycle Sludge (mg/l) 22200 BOD lnfluent, Unfiltered (mg/l) 120 BOD lnfluent, Filtered (mg/l) 64 Total Suspended Solids in lnfluent (mg/l) I53 Nominal Residence Time in Aeration Basin (hours) 1.34 Nominal Residence Time in Initial Liquid Stage (hours) 0.17 Recycle Volume, of lnfluent Flow 8.9 F,/M,,, lnitial Liquid Stage 4.6 F,/M,,, Total Aeration Basin 0.57 BOD Effluent, Unfiltered (mg/l) 22 BOD Effluent, Filtered (mg/l) 8.5 BOD, Removed, Unfiltered ln- Filtered Out 93 Temperature, C l9.5 Maximum Oxygen Uptake Rate,

mg Oxygen/g MLVSS/hr. 75 Activity Coefficient (a) 0.9 Oxygen Consumed/1.5 BOD Soluble Removed, wt/wt 1.0 Oxygen Consumed/BOD, Total Removed, wt/wt 0.8 SV! (ml/g MLVSS) 45 ZSV (ft/hr) 21 Practice of the invention does not require any special means for incorporation of oxygen into the mixed liquor comprising the BOD-containing aqueous waste and recycled activated sludge. Thus, one may employ the conventional means for introducing the oxygencontaining gas and admixing the same with the liquid, such as by diffusers or spargers below the liquid level with or without auxiliary mechanical circulating and agitating means, or by the use of surface aerators. Where the oxygen uptake rate in a treating zone exceeds the capacity to transfer oxygen by use of atmospheric air, the air may be supplemented or enriched by pure oxygen. For example, to satisfy the oxygen demand of a mixed liquor having a total oxygen uptake rate in excess of 100 mg/l/hr, the use of an oxygenenriched aeration gas (in excess of 21% 0 by volume) is recommended.

We claim:

1. In the operation of an activated sludge system having at least two successive aeration stages, the method of inhibiting proliferation of filamentous biomass which comprises: aerating in an initial stage a mixed liquor comprising BOD-containing aqueous waste in admixture with activated sludge while controlling at least one variable selected from the group consisting of initial stage influent residence time, activated sludge recycle rate, and initial stage MLVSS concentration such that the ratio of soluble food to active biomass as determined by the expression F,/M, is at' least 4 and there is maintained in the mixed liquor during said initial stage a dissolved oxygen content numerically equal in parts per million to at least 2 or one-tenth the numerical value of the ratio F,/M,,, whichever is the greater,

wherein F, is equal to 1.5 times the weight of soluble BOD charged per day to the mixed liquor and M is equal to the quantity of MLVSS in the mixed liquor in said stage multiplied by an activity coefficient determined by the ratio of the maximum specific oxygen uptake rate of the volatile suspended solids (VSS) under aeration to the predetermined reference maximum specific oxygen uptake rate MSOUR for the same temperature, MSOUR being determined by the equation:

in MSOUOR 33.92 8640/T(K).

2. The method in accordance with claim 1 wherein the mixed liquor is one having an oxygen demand characterized by a total oxygen uptake rate in excess of mg/l/hr, and said aeration is effected with the use of oxygen-enriched gas.

3. The method of treating BOD-containing aqueous waste which comprises aerating a mixed liquor comprising such waste in admixture with activated sludge in an initial liquid treating stage while controlling at least one variable selected from the group consisting of initial stage influent residence time, activated sludge recycle rate, and initial stage MLVSS concentration to maintain a high ratio of food to biomass and a dissolved oxygen content in the mixed liquor of such initial stage numerically equal in parts per million to at least 2 or one-tenth the numerical value of the ratio F lM whichever is the greater, thereby promoting selective production and growth of dense, rapid-settling microorganisms and thereafter transferring the so-aerated mixed liquor to at least one additional stage of aeration to substantially complete the oxidation and assimilation of remaining BOD values; said high ratio of food to biomass in said initial stage being at least equal to a value expressed by the ratio F,/M,, wherein F, is equal to 1.5 times the weight of soluble BOD charged per day to said initial aeration stage and M is equal to the weight of MLVSS in said stage times the coefficient 11,01 being the ratio of the oxygen uptake rate of the MLVSS present in said mixed liquor divided by the reference maximum specific oxygen uptake rate MSOUR at the treating temperature, MSOUR being determined by the equation:

In MSOUR 33.92 8640/T(K).

4. The method in accordance with claim 3 wherein the ratio F,/M,, in said initial liquid treating stage is at least 8.

5. The method in accordance with claim 4 wherein said aeration is effected by use of an aeration gas having an oxygen content in excess of 21% by volume.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. I 3,864,246

DATED 3 February 4, 1975 r v Toms) Jeremiah P. Casey; Curtis S. McDowell;

Marshall L. Spector and Alan J. Zupko It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 3, line 53, "commoly" should read commonly In column 4, line 36, after "specific" insert oxygen In column 12, line 16 "MSOUOR" should read MSOUR In the twelfth column of Table 1, below the heading 400 and 5. 7:

" 1.80 should read 1.68

In Table 5, lines 7 and 9 below the table heading (each occurrence) "Nominal Residence Time" should read Influent Detention Time Signed and Qcaled this First D y March 1977 [SEAL] A rtes r.-

RUTH C. MASON C. MARSHALL DANN Arresting ()j'jr'cer (mnmissinm' nj'Parenrs and Trademarks 

1. IN THE OPERATION OF AN ACTIVATED SLUDGE SYSTEM HAVING AT LEAST TWO SUCCESSIVE AERATION STAGES, THE METHOD OF INHIBITING PROLIFERATION OF FILAMENTOUS BIOMASS WHICH COMPRISES: AERATING IN AN INITIAL STAGE A MIXED LIQUOR COMPRISING BODCONTAINING AQUEOUS WASTE IN ADMIXTURE WITH ACTIVATED SLUDGE WHILE CONTROLLING AT LEAST ONE VARIABLE SELECTED FROM THE GROUP CONSISTING OF INITIAL STAGE INFLUENT RESIDENCE TIME, ACTIVATED SLUDGE RECYCLE RATE, AND INITIAL STAGE MLVSS CONCENTRATION SUCH THAT THE RATIO OF SOLUBLE FOOD TO ACTIVE BIOMASS AS DETERMINED BY THE EXPRESSION F2/MZ IS AT LEAST 4 AND THERE IS MAINTAINED IN THE MIXED LIQUOR DURING SAID INITIAL STAGE A DISSOLVED OXYGEN CONTENT NUMERICALLY EQUAL IN PARTS PER MILLION TO AT LEAST 2 OR ONE-TENTH THE NUMERICAL VALUE OF THE RATIO F2/MA, WHICHEVER IS THE GREATER, WHEREIN FS IS EQUAL TO 1.5 TIMES THE WEIGHT OF SOLUBLE BOD5 CHARGED PER DAY TO THE MIXED LIQUOR AND MA IS EQUAL TO THE QUANTITY OF MLVSS IN THE MIXED LIQUOR IN SAID STAGE MULTIPLIED BY AN ACTIVITY COEFFICIENT DETERMINED BY THE RATIO OF THE MAXIMUM SPECIFIC OXYGEN UPTAKE RATE OF THE VOLATILE SUSPENDED SOLIDS (VSS) UNDER AERATION TO THE PREDETERMINED REFERENCE MAXIMUM SPECIFIC OXYGEN UPTAKE RATE MSOUR FOR THE SAME TEMPERATURE, MSOUR BEING DETERMINED BY THE EQUATION: 1N MSOUOR = 33.92 - 8640/T(*K).
 2. The method in accordance with claim 1 wherein the mixed liquor is one having an oxygen demand characterized by a total oxygen uptake rate in excess of 100 mg/1/hr, and said aeration is effected with the use of oxygen-enriched gas.
 3. The method of treating BOD-containing aqueous waste which comprises aerating a mixed liquor comprising such waste in admixture with activated sludge in an initial liquid treating stage while controlling at least one variable selected from the group consisting of initial stage influent residence time, activated sludge recycle rate, and initial stage MLVSS concentration to maintain a high ratio of food to biomass and a dissolved oxygen content in the mixed liquor of such initial stage numerically equal in parts per million to at least 2 or one-tenth the numerical value of the ratio Fs/Ma, whichever is the greater, thereby promoting selective production and growth of dense, rapid-settling microorganisms and thereafter transferring the so-aerated mixed liquor to at least one additional stage of aeration to substantially complete the oxidation and assimilation of remaining BOD values; said high ratio of food to biomass in said initial stage being at least equal to a value expressed by the ratio Fs/Ma wherein Fs is equal to 1.5 times the weight of soluble BOD5 charged per day to said initial aeration stage and Ma is equal to the weight of MLVSS in said stage times the coefficient Alpha , Alpha being the ratio of the oxygen uptake rate of the MLVSS present in said mixed liquor divided by the reference maximum specific oxygen uptake rate MSOUR at the treating temperature, MSOUR being determined by the equation: 1n MSOUR 33.92 - 8640/T(*K).
 4. The method in accordance with claim 3 wherein the ratio Fs/Ma in said initial liquid treating stage is at least
 8. 5. The method in accordance with claim 4 wherein said aeration is effected by use of an aeration gas having an oxygen content in excess of 21% by volume. 